1. Field of the Invention
The invention relates to an overvoltage protection device with a first electrode which has a first arcing horn, with a second electrode which has a second arcing horn, with an air-breakdown spark gap which is active between the arcing horns, and with a housing which accommodates the electrodes, the two arcing horns being made and arranged relative to one another such that they diverge from a lower ignition area each to their outer ends, so that the air-breakdown spark gap widens to the outside, proceeding from the ignition area.
2. Description of Related Art
Electrical, but especially electronic measurement, control and switching circuits, mainly also telecommunications means and systems, are sensitive to transient overvoltages, as can occur especially by atmospheric discharges, but also by switching operations and short circuits in power supply grids. This sensitivity has increased to the extent that electronic components, especially transistors and thyristors, are used; mainly integrated circuits which are being increasingly used are greatly endangered by transient overvoltages.
To protect electrical, but especially electronic measurement, control and switching circuits, mainly also telecommunications means and systems, against transient overvoltages, generally, wherever electronic circuits are used, overvoltage protection devices have been developed and used for more than twenty years.
One important component of an overvoltage protection device is at least one spark gap which operates at a certain overvoltage, the operating voltage, and thus, prevents overvoltages which are greater than the operating voltage of the spark gap from occurring in the area protected by this overvoltage protection device.
It was mentioned at the start that the type of overvoltage protection device to which the invention is directed has two electrodes and an air-breakdown spark gap which acts between the electrodes or their arcing horns. In addition to overvoltage protection devices with an air-breakdown spark gap, there are overvoltage protection elements with an air-flashover spark gap in which a creeping discharge occurs upon response. Overvoltage protection devices with an air-breakdown spark gap, as compared to overvoltage protection devices with an air-flashover spark gap, have the advantage of higher current carrying capacity, but the disadvantage of a higher and not especially constant operating voltage. Therefore, various overvoltage protection devices with an air-breakdown spark gap have been proposed which have been improved with reference to the operating voltage. Here, in the area of the arcing horns or the air-breakdown spark gaps which act between the arcing horns, ignition aids have been accomplished in various ways, for example, such that there was at least one ignition aid between the arcing horns which triggers a creeping discharge and which projects partially into the air-breakdown spark gap, the ignition aid being made in the manner of a bridge of plastic.
The overvoltage protection device upon which the present invention is based is known from German Patent DE 44 02 615 C 2 and corresponding U.S. Pat. No. 5,604,400. The known overvoltage protection device has two narrow electrodes which are each made angled and each have an arcing horn and a terminal leg bent away from it. The known overvoltage protection device is characterized in that, based on the electrodes which extend more or less in one plane, it has only a small overall width. Furthermore, the known overvoltage protection device, in any case, when operated for the first time, has a high current carrying capacity and high network follow current extinction capacity. However, problems always arise when an arc jams at one location; this can lead to electrode damage.
Furthermore, German Patent DE 196 04 947 C 1 discloses an overvoltage protection device which is made essentially rotationally symmetrical. The two electrodes are identical and each have arcing horns in the shape of truncated cones, and an ignition bridge for triggering a creeping discharge is located between the two end faces of the arcing horns. This known overvoltage protection device is likewise characterized by high current carrying capacity and a high network follow current extinction capacity. However, the disadvantage is that this known overvoltage protection device has a relative high overall height. Another disadvantage is that, when the spark gap operates, considerable forces act on the extremely sensitive ignition bridge; this can lead to the ignition bridge being damaged.
A primary object of this invention is, therefore, to devise an overvoltage protection device of the type mentioned above which is characterized by a high current carrying capacity and high network follow current extinction capacity with an overall height which is as small as possible.
The overvoltage protection device as in accordance with the invention, in which the aforementioned object is achieved, first of all, is essentially characterized by the first arcing horn being made in the shape of a truncated cone and the second arcing horn being located concentrically around the first arcing horn. In accordance with the invention, the first electrode with the first arcing horn represents a middle electrode, while the second electrode with the second arcing horn represents an outer electrode. The concentric arrangement of the second arcing horn around the first arcing horn yields a horn spark gap which runs entirely or partially around the center axis of the two electrodes. This yields a peripherally closed spark gap when the second arcing horn is made circular and the surface of the second arcing horn facing the first arcing horn is made in the shape of a truncated funnel.
However, basically, it is also possible to make the second arcing horn such that it has a plurality of arcing horn segments which are spaced apart from one another and arranged concentrically around the first arcing horn. One especially preferred embodiment of the overvoltage protection device according to the invention is provided with a second arcing horn which is made in the shape of a ring segment and the surface of the second arcing horn facing the first arcing horn is made in the shape of a truncated funnel. Preferably, the second arcing horn extends over an angle of roughly 210xc2x0 to 270xc2x0, especially over an angle of roughly 240xc2x0. The embodiment of the overvoltage protection device in accordance with the invention in which the second arcing horn is made in the shape of a ring segment is one in which there is a horn spark gap which extends only partially around the center axis of the two electrodes.
In the overvoltage protection device according to the invention, the horn spark gap which runs only partially around the center axis of the electrodes is characterized by a very high current carrying capacity and a, likewise, high network follow current extinction capacity. Furthermore, the concentric arrangement of the second arcing horn around the first arcing horn yields a very low overall height of the overvoltage protection device and a simple structure.
A preferred embodiment of the overvoltage protection device as in accordance with the invention, furthermore, has at least one ignition bridge of electrically insulating material in the ignition area between the two arcing horns. Via the ignition bridge, a creeping discharge is triggered which, once initiated, at a relatively constant, low operating voltage, leads to ignition of the air-breakdown spark gap with a relatively high current carrying capacity. Since, in the overvoltage protection device of the present invention, in contrast to the overvoltage protection device known from German Patent DE 196 04 947 C 1, the spark gap is open in the axial direction instead of the radial direction, the mechanical forces acting on the extremely sensitive ignition bridge are minimized when the spark gap is operated. So that a creeping discharge is possible anywhere on the entirely or partially peripheral horn spark gap, the ignition bridge is made ring-shaped; therefore, like the second arcing horn, it is arranged concentrically around the first arcing horn. Preferably, the ignition bridge is made of a plastic which evolves an arc extinguishing gas when heated. This improves the extinction behavior of the overvoltage protection device after operating.
The overvoltage protection device in accordance with the invention, as is conventional, has a first terminal area and a second terminal area. Preferably, it is provided in this connection that the first terminal area and the second terminal area are located on the same side of the overvoltage protection device. This yields the advantage that, in the case of operation, the electromagnetic forces which act on the arc support its continued movement from the ignition area into the widened horn area.
The last described embodiment of the overvoltage protection device according to the invention, preferably, also has holes and/or through openings in the terminal areas of the electrodes and there are terminal elements which extend above and beyond the terminal areas, specifically a PE terminal element and a potential terminal element, and preferably in one terminal area, preferably in the terminal area provided for the potential terminal element, there are two connection possibilities. Therefore, in this embodiment, there need not necessarily be terminal elements which are integrated into the overvoltage protection device and which project from the latter, so that overvoltage protection device can have a disk or pot shape, as it were. Of course, in this case, the electrodes must be connected to the terminal elements which can be inserted, for example, screwed in, in the region of the terminal areas. Otherwise, because there are through openings in the electrodes, venting can be easily obtained.
If, in the overvoltage protection device of the invention, as described last, the two terminal areas, and thus the two terminal elements, are on the same side, it is recommended that there be at least one bridge, which is made preferably undulating, on the outside in the region of the terminal areas to increase the creepage distance between the terminal areas. In particular, when one of the two terminal areas has two connection possibilities, it is a good idea to make the bridge undulating and to place it between the terminal areas.
It has already been stated above that one especially preferred embodiment of the overvoltage protection device in accordance with the invention has the second arcing horn made in the shape of a ring segment. In this embodiment, it is recommended that the second arcing horn be aligned to the second terminal area, and thus, to the potential terminal element. This means then that the recess which the second arcing horn has is aligned relative to the first terminal area, and thus, to the PE terminal element.
The preferred embodiment of the overvoltage protection device according to the invention which was described last preferably also has the potential terminal element on the side of the two electrodes which is opposite the PE terminal element, and preferably, at least the second electrode, and thus the second arcing horn, but preferably both electrodes, and thus the two arcing horns, are made and arranged symmetrically with reference to the PE terminal element.
Otherwise, a further improved network follow current extinction capacity in the overvoltage protection device of the invention arises by the housing being encapsulated pressure-tight. The pressure-tight encapsulation also makes it possible to fill the housing interior with an arc extinguishing gas; this can contribute further to improvement of the network follow current extinction capacity.
In the overvoltage protection device in accordance with the invention, the housing can have a housing bottom part and a housing cover, then preferably, the housing bottom part and the housing cover can be screwed and/or cemented to one another. Otherwise, it is provided for reasons of touch protection that the housing bottom part and the housing cover do not make contact with the first electrode or the second electrode anywhere; preferably, between the first electrode and the second electrode, on the one hand, and the housing bottom part and the housing cover, on the other hand, there is at least one insulating part and/or an air gap. If, in the overvoltage protection device according to the invention, as described above, in the ignition area between the two arcing horns, there is at least one ignition bridge for triggering a creeping discharge, then it is recommended that the ignition bridge be made in one piece with the insulating part. This minimizes the number of components of the overvoltage protection device of the invention.
Otherwise, it is recommended that the overvoltage protection device of the present invention has, to further improve the current carrying capacity and the network follow current extinction capacity, a baffle plate at a distance from the ends of the arcing horns, so that an arc runs against the baffle plate in the case of operation of the overvoltage protection device. By using a baffle plate which, like the electrodes, preferably, is made of a copper-tungsten alloy, damage of the possibly plastic housing can be precluded. Furthermore, the baffle plate influences energy conversion in the gas space. The arc is divided specifically into three parts by the baffle plate. Two arc areas and a current area form in the baffle plate. This yields two anode-cathode falls and also an overall reduced arc, since some of the current path runs in the baffle plate and produces no plasma.
In order to have available a so-called xe2x80x9cactivexe2x80x9d ignition aid in conjunction with the overvoltage protection device of the invention, in the area of the air-breakdown spark gap and in addition to it, there is an ignition spark gap which can be actively ignited, specifically depending on the stipulated ignition voltage. Preferably, there is an ignition spark gap between an ignition electrode and one of the two electrodes, so that only one additional electrode, specifically the ignition electrode, is necessary. The ignition electrode can be easily built by its being formed by the housing and/or the baffle plate, and it then goes without saying that the housing and/or the baffle plate each is made of an electrically conductive material. Otherwise, in this connection, it must be ensured that neither the housing nor the baffle plate are in electrical contact with one of the electrodes. Of course, when there is an xe2x80x9cactivexe2x80x9d ignition aid, there must be available a corresponding ignition circuit for the ignition spark gap.
It was stated above that, in the overvoltage protection device in accordance with the invention, a further improved network follow current extinction capacity can arise by the housing being encapsulated in a pressure-tight manner. Especially in the embodiment of the overvoltage protection device of the invention described above, in which the second arcing horn is made in the shape of a ring segment instead of being ring-shaped, it is recommended that the housing be provided with pressure equalization openings. If in the overvoltage protection device of the invention the housing is comprised of a housing bottom part and a housing cover, it is recommended that the housing bottom part and/or the housing cover, preferably the housing bottom part, be provided on its edge facing the housing cover with preferably radially running slots which act as pressure equalization openings.